gdb: remove discard_psymtab function

This function does not add much value, compared to calling the method on
the psymtab_storage object directly.

gdb/ChangeLog:

	* psympriv.h (discard_psymtab): Remove.
	* dbxread.c (dbx_end_psymtab): Update.
	* xcoffread.c (xcoff_end_psymtab): Update.